Despite the proliferation of adolescent e-cigarette use (i.e., vaping) and evidence of associations of vaping with cigarette initiation, research on whether vaping increases the risk of progression to regular, dependent smoking patterns and on factors that underlie these risk pathways are scant. Consequently, there are limited data to guide interventions to protect the growing population of adolescent vapers from transitioning to smoking. We hypothesize that a pro-tobacco social environment (e.g., lower socioeconomic environment in which smoking may be more accepted, proximity to tobacco retailers, having friends who smoke) may amplify the risk of transition from vaping to regular smoking and dependence, particularly among youth with a positive sensory-pharmacological response to their initial vaping experience who may therefore be more inclined to experiment with other tobacco products. This project will leverage data from two ongoing longitudinal cohorts to study vaping as a predictor of risk of smoking progression and dependence in adolescents aged 14-20 (Pooled N~3700; Aim 1a) and the role of the social environment and early vaping experience as moderators of this risk pathway (Aim 1b); we will also examine whether these associations differ by gender and age (Aim 1c). In a parallel project, we will conduct qualitative interviews with youth in these cohorts to uncover novel factors that influence this transition (N=60; Aim 2). Results from Aims 1 and 2 will be used to generate a new psychological survey to assess a novel construct of `susceptibility to transition from vaping to smoking.' We will conduct an initial validation study in a subset of youth vapers from both cohorts (N=200; Aim 3). Findings will be informative in (1) elucidating the possible harms that vaping may pose to population health by increasing youth smoking, which is important to guide development of policies and regulation of youth vaping; and (2) guiding the development of prevention strategies to address the social environment and alter the vaping experience to mitigate the risk of smoking among the rapidly increasing population of vaping adolescents. I enter this project with strong quantitative skills as an epidemiologist and growing experience in tobacco research. This K01 is critical for me to develop expertise in areas that will support my long-term goals to become an independent multi-disciplinary investigator of the intersecting environmental and intrapersonal determinants of adolescent tobacco and other drug use. The training proposed will help me to develop expertise in a) adolescent developmental psychopathology of addiction, b) qualitative research methodologies, and c) survey design and psychometric testing of survey items, with mentorship to additionally refine my skills in application of my research to inform policy and prevention efforts. I have worked closely with my mentors (selected based on their expertise in each training area) to develop an ambitious but feasible training plan that incorporates expert mentoring, directed readings, coursework and seminars, scientific meetings, and continued development of manuscript and grant writing skills.